PROBLEM TO BE SOLVED: To make it possible to accurately measure a water level by using an optical fiber water level gauge even when there is no difference between air temperature and water temperature.SOLUTION: Optical fiber of an optical fiber water level gauge is wound onto each of a first sensing part provided to an outer peripheral part of a heat insulating material and a second sensing part provided to an inner peripheral part of the heat insulating material. Temperature distribution of the first sensing part in a height direction and the temperature distribution of the second sensing part in the height direction are measured. When air temperature and water temperature are the same, a water level is detected from time change of the temperature distribution of the second sensing part in the height direction.SELECTED DRAWING: Figure 5
